1. Field of the Invention
This invention relates to a light measuring device for a camera having the photometric range thereof arranged to be shiftable.
2. Description of the Prior Art
The light measuring methods generally adopted for a camera can be roughly divided into an averaging photometry which measures the light of relatively wide area and a spot photometry which measures the light of a relatively narrow area. In the averaging photometry, since the average brightness of a wide area of an object to be photographed is measured, a stable light measurement value can be obtained when the object does not have much variations in local brightness. The averaging photometry, therefore, is suited for general automatic exposure photography. However, in the event of extreme difference in brightness between a background and a photographing subject, like in the case of a back-light shot, the optimum value of light measurement is very difficult to obtain. In the case of the spot photometry on the other hand, the optimum value of light measurement can be obtained for a photographing subject irrespective of the ambient brightness since light measurement is limited to a narrow range within the object to be photographed. While this is an advantage, the spot photometry is inapposite to general automatic exposure photography because the light measurement value varies to a great extent with variations in the object brightness.
Such being the merits and demerits of the averaging and spot photometric modes, it is desirous to arrange a light measuring device to permit selective shifting from one photometric range to another or between these different photometric modes according to the photographing conditions. In the light of this, there have been proposed various methods for effecting such switch-over. The conventional known photometric range shifting methods include: A method in which the photometric range is shifted by mainly shifting the position of a photometric optical system to vary the imaging magnification thereof; and another method in which a light receiving photo-sensitive element which has its light receiving area divided into a plurality of photometric regions is used and the photometric range is shifted by selecting, according to the photographing conditions, the photometric regions of the light receiving surface of the element on which an object image is formed. However, the former method of shifting the photometric optical system causes the light measuring device to become complex and results in a larger size thereof. The latter method permits simple and compact structural arrangement as the photometric regions of the light receiving surface can be electrically switched over from one region to another. The latter method, however, presents a problem in that good photometric sensitivity distribution is not readily obtainable.
FIG. 1 of the accompanying drawings schematically shows the basic arrangement of the photometric optical system of the conventional single-lens reflex camera. An object image formed on a focusing plate 1 is arranged to be approximately formed on the light receiving surface 4a of a photo-sensitive element 4 by an image forming lens 3 through a penta-Dach prism 2. The light receiving surface 4a of the photo-sensitive element 4 is arranged as shown by way of example in FIG. 2. As shown, the light receiving surface is divided into a center region A and a peripheral region B.
FIG. 3 is a development view showing the image forming state of the photometric optical system shown in FIG. 1. The region A of the light receiving surface 4a corresponds to a relatively narrow range A1 located in the center part of the focusing plate 1 while the region B corresponds to a relatively wide range B1 on the focusing plate 1. Therefore, the spot photometry is performed by the output of the region A of the photo-sensitive element 4 and the averaging photometry by the output of both the regions A and B of the photo-sensitive element.
The photometric optical system, which is arranged as described above, has a photometric sensitivity distribution as shown in the graph of FIG. 4. In this graph, an axis X shows distances from a center O of the focusing plate 1 while an axis Y shows relative sensitivity obtained at each distance. A curve "a" represents photometric sensitivity distribution obtained by the spot photometry and a curve "b" photometric sensitivity distribution obtained by the averaging photometry.
As apparent from FIG. 4, a shortcoming of the conventional arrangement lies in that the photometric sensitivity distribution flattens over the whole light receiving surface of the photo-sensitive element in the case of the averaging photometry. As a result, the light measuring value becomes excessively responsive to the ambient brightness around the object to be photographed and thus becomes excessively affected by the background brightness for an ordinary object to be photographed. Generally, it is preferable for the averaging photometry to have the so-called center emphasizing photometric sensitivity distribution in which the sensitivity for the peripheral area of the object gradually lowers according as the distance of the peripheral area from the center part of the object increases.